Crate BV120H Retube

[Teleman55]

A customer brought me this amp to retube and wants to use EL34's in place of the 6L6 output tubes.I don't know if this amp will support the current demands of EL34's.I need some advice on wether this is a safe move,I know for some amps this is not recommended.

[Enzo]

As with any amp you want to do this, the EL34 heaters draw quite a bit more vurrent than 6L6. The thing to do is install the EL34s and fire up the amp, leaving it in standby so no B+ gets to the tubes. If teh heater voltage stays up around 6.3VAC, then it will likely be OK as far as that aspect. If it sags below, then you shouldn't do it.

If the heaters check out, then look at the power tube PC board foil tracery. Unlike 6L6, the EL34 MUST have pin 1 grounded or at least connected to the cathode pin 8. It might already be connected that way. Or pin 1 mught be open and a simple jumper wire from pin 1 to 8 will suffice. But on a lot of amps, since pin 1 is not used on the 6L6, the traces might connect the pin 1 to something else. If that is the case, you'd have to cut traces around pins 1 and deal with it. And check ALL FOUR tube sockets. Just because a couple of them are wired OK, doesn't mean they all are. MAKE SURE. The schematic is draw to make me think that pin 1 is already wired for this

If THAT is OK, then I would increase the screen resistors from the 470 ohms up to 1k ohms, 5 watt.

Finally, the bias. Looks to me like the bias on this amp is adjustable from about 48 volts up to about 64 volts. That may be too high to get the EL34s up to a good warm glow. To lower the bias voltage range, I would lower the value of R105 from the stock 15k to something like 6.8k or something. 5.6k resilts in a low end of 28volts and 6.8k results in about 32 volts minimum. Even the 5.6k still allows the adjustment up to 60 volts, so the range is still plenty.

[Teleman55]

Thank Enzo,I like the idea of checking the heater voltage.The info is much appreciated.